1. Field of the Invention
The present invention relates to nitride semiconductor light-emitting devices including GaN-based light-emitting devices.
2. Description of the Related Art
Recently, the Blue-ray Disc has been released as a coming-generation high-density optical disk. The Blue-ray Disc uses a semiconductor laser, which emits blue-violet light, as a light source. In such a semiconductor laser a gallium nitride (GaN)-based III-V compound semiconductor is used as a semiconductor material. In light of the future development the Blue-ray Disc calls for high-density and high-speed recording capability. For this reason a GaN-based semiconductor laser exhibiting high optical output and high reliability will be needed for use with the Blue-ray Disc.
In prolonging the lifetime of such a GaN-based semiconductor laser, reduction in power consumption and in dislocation density is critical. According to the teaching of S. Tomiya et. al, Phys. Stat. Sol.(a), 188(2001)69. (non-patent document 1) for example, the reduction in power consumption has a close correlation with prolongation of the lifetime of a GaN-based semiconductor laser. S. Nagahama et. al, Jpn. J. Appl. Phys., 39(2000)L647 (non-patent document 2) and M. Ikeda et. al, Phys. Stat. Sol.(a), 194(2002)407 (non-patent document 3) have suggested that the reduction in dislocation density is effective in prolongation of lifetime. Both of non-patent documents 2 and 3 use a sapphire substrate, grow a GaN film on the sapphire substrate, form an insulating film on a portion of the GaN film, and grow a GaN film again. The GaN film grown again begins to grow selectively from a region not formed with the insulating film and then grows laterally to protrude over the insulating film. This technique is called epitaxial lateral overgrowth (ELO.)
In such an ELO region, propagation of threading dislocation caused by the difference in physical constant between sapphire and GaN toward the surface of the GaN film is suppressed and, hence, a low dislocation-density region is formed. The dislocation density of such a low dislocation-density region is about 106 cm−2. On the other hand, a high dislocation-density region out of the ELO region has a dislocation density of about 108 cm−2, which is higher than that of the low dislocation-density region by about two orders of magnitude. For the pursuit of a prolonged lifetime under high-temperature and high-output conditions, however, the dislocation density of about 106 cm−2, which is presently attainable, is insufficient and a further reduction in dislocation will be needed. Since a sapphire substrate has an insulating property, electrodes of a semiconductor laser cannot be disposed on the substrate side. For this reason, a GaN-based laser formed on the sapphire substrate utilizing the ELO technique has p-electrode and n-electrode both disposed on the GaN side. This structure increases the device size, hence, decreases the number of devices to be obtained from a single substrate. Further, the structure calls for a complicated fabrication process, which results in problems of decreased yield and increased fabrication cost.
In attempt to solve the aforementioned problems, an electrically conductive (n-type) GaN substrate has recently begun being fabricated and released. In each of Japanese Patent Laid-Open Publications No. 2003-124572 (patent document 1) and No. 2003-133649 (patent document 2) for example, there is disclosed a method of fabricating a self-contained GaN substrate, which includes: growing a thick GaN film on a support substrate of sapphire for example by utilizing the aforementioned ELO technique; removing the support substrate by polishing; and slicing and mirror-finishing the thick GaN crystal film. Since the ELO technique is utilized in the fabrication of such a GaN substrate, the substrate has a low dislocation-density region and a high dislocation-density region. The low dislocation-density region has a dislocation density lowered to about 105 cm−2. Further, attempts have been made to fabricate a GaN-based laser on such a GaN substrate. According to the teaching of 0. Matsumoto et. al, Extended Abstracts of the 2002 Int. Conf. on Solid State Devices and Materials, pp. 832-833 (non-patent document 4), a GaN-based laser fabricated on a GaN substrate having a low dislocation density (about 3×105 cm−2) is expected to enjoy a considerably prolonged lifetime estimated to be about 100,000 hours.
In an attempt to solve problems, including substrate warpage and crystalline defect, common to all the types of substrates, Japanese Patent Laid-Open Publication No. 2003-110197 (patent document 3) has disclosed a technique wherein a first buffer layer of GaN having plural trenches at its surface is formed directly on a substrate and then a second buffer layer of GaN doped with magnesium (Mg) is formed on the first buffer layer.